1. Field of the Invention
The present invention relates to an inverter device, a plasma generating device which uses the inverter device as a power source, a sheet-member reforming device using the plasma generating device, and an alternating-current voltage output method.
2. Description of the Related Art
A switching regulator or an inverter device is used to supply high voltage to various devices, such as a discharge tube for large plasma display and a plasma generating device.
In general, a switching regulator or an inverter device with the output power value of about a few watts is commonly used; however, in a plasma generating device or the like, an inverter device with the output voltage (effective value) of a dozen kilovolts and alternating-current (AC) output power value of tens of watts or higher is used.
It is said that as a plasma discharge, for example, an atmospheric-pressure plasma discharge is generally generated by the application of a high voltage of six kilovolts or higher under an ordinary pressure condition. As a means of realizing the atmospheric-pressure plasma discharge, there are a dielectric-barrier discharge, a silent discharge, and a corona discharge in the atmosphere, etc.
For example, to perform a surface reforming process for reforming the surface of a sheet member such as a printing sheet, an atmospheric-pressure plasma generating device is used, and an inverter device which generates AC high voltage is used as a power source of the atmospheric-pressure plasma generating device.
Japanese Laid-open Patent Publication No. 2011-57442 has proposed a reforming device for reforming such a sheet member. A plasma generating device in the reforming device includes a discharge unit in which a round bar-like discharge electrode 121, which extends in a direction perpendicular to the plane of paper, and a flat plate-like counter electrode 122 are opposed across a dielectric (an insulator) 123 as shown in FIG. 8. The counter electrode 122 is grounded.
AC high voltage output from a high-voltage power source 100 is applied between the discharge electrode 121 and the counter electrode 122, thereby an atmospheric-pressure plasma discharge (such as a dielectric-barrier discharge by a creeping discharge) indicated by dashed lines is generated, and plasma is formed.
The contact area of this plasma formed by the creeping discharge with a processed surface of a sheet member (not shown) when the sheet member is conveyed along the surface of the dielectric 123 is large; therefore, the occurrence of irregularity in reforming is suppressed, and reforming uniformity is ensured with high accuracy. In general, an inverter device is used in the high-voltage power source 100.
Such a surface reforming device is applied in various fields; as an example, the surface reforming device is used in a pre-processing system of a printer. By performing a surface reforming process on a printing sheet before an image is printed on the printing sheet, adhesion and permeability of ink to the printing sheet can be controlled, and therefore color development of ink can be improved. Consequently, it is possible to expect effect of reduction in amount of ink used.
The surface reforming process on a sheet has to be performed immediately before printing, and has to be incorporated into the flow of a printing operation. Therefore, depending on the productivity of the printer, i.e., conditions of printing speed and printing sheet size, etc., the plasma generating device is required to have very high surface reforming ability.
There are several means for obtaining high processing capacity; however, to improve the processing effect by increasing the time and number of times of surface processing through discharge, the effective way is to increase the area of discharge from the plasma generating device. However, increasing the discharge area causes an increase in load; therefore, it is necessary to increase the output from an inverter device which applies high voltage between electrodes.
To obtain high-voltage high output stably by means of an inverter device, boosting by a large transformer or the like is needed.
For this purpose, if the turns ratio between excitation winding and output winding of a transformer composing an inverter device is increased, there occur problems such as an increase in loss due to increase in winding resistance or line capacity, a limitation on an available frequency band, magnetic saturation of the core, and heat generation of the core and windings.
To cope with such problems, Japanese Laid-open Patent Publication No. 2012-186984 has proposed an inverter device composed of multiple transformers having the same characteristics, wherein an excitation current is applied to respective excitation windings of the transformers to excite the excitation windings, and respective output windings are connected in series or in parallel with one another, thereby obtaining a higher-power AC output than a conventional inverter device.
However, to increase the area of discharge from a plasma generating device, it is necessary to increase the area of a counter electrode and install a plurality of discharge electrodes opposed to the counter electrode.
With the increase in number of discharge electrodes, if voltage is applied to the discharge electrodes by a single inverter device, the distance from an output terminal of the inverter device varies according to the position of the discharge electrodes. That is, with increasing distance of a power supply path from the output terminal of the inverter device, an inductance component generated in the power supply path increases, and, as shown in FIG. 10, a time lag Δt due to phase delay occurs in a voltage waveform. This causes a potential difference ΔV between discharge electrodes applied with the voltage, and therefore, unnecessary discharge may be generated between the discharge electrodes.
Accordingly, as a means for increasing output, as shown in FIG. 9, multiple (in FIG. 9, two) inverter devices 101 and 102 can be arranged side by side and separately apply output voltage to respective multiple discharge electrodes 121A and 121B composing a discharge unit 120 of a plasma generating device. A counter electrode 122 and a dielectric (an insulator) 123 are the same as those shown in FIG. 8.
This enables respective distances of power supply paths from output terminals of the inverter devices 101 and 102 to the discharge electrodes 121A and 121B to be about the same, i.e., can ensure that almost there is almost no difference in distance.
However, when the side-by-side inverter devices 101 and 102 are operated independently, respective output voltages are out of phase as shown in FIG. 11; therefore, a difference in instantaneous value is caused by a time lag Δt between output voltage waveforms of the inverter devices 101 and 102. When this has caused a large potential difference ΔV between the discharge electrodes 121A and 121B, a discharge is generated between the discharge electrodes, and this may cause damage to the discharge electrodes and an increase in power loss.
Actual output voltage waveforms of the inverter devices 101 and 102 are not sinusoidal; however, for the sake of simplification of concepts, output voltage waveforms of the inverter devices 101 and 102 are shown as a sinusoidal waveform in FIGS. 10 and 11, where time is plotted on the horizontal axis and voltage is plotted on the vertical axis.
When there is a phase shift in a direction of time between the two voltage waveforms, a potential difference ΔV is generated according to a shift amount Δt of the phase shift. In the worst case, when the two voltage waveforms are shifted in phase by τ (180 degrees), the polarities of peaks of the two voltage waveforms are opposite to each other, thereby about a potential difference corresponding to the double of sine-wave amplitude is generated.
If there is a similar shift between output voltages of two inverter devices with the output voltage of tens of kilovolts, a potential difference between output voltages of the two inverter devices is tens of kilovolts at most.
Therefore, unnecessary discharge unrelated to plasma generation for surface reforming, which is the original purpose, may be generated between the discharge electrodes. The discharge generated between the discharge electrodes at this time may become an arc discharge which is different from a dielectric-barrier discharge that generates plasma, and this may incur a risk such as damage to an electrode in addition to power loss due to the discharge.
Furthermore, even when there is no shift (phase shift) in the direction of time between output voltage waveforms of two inverter devices, if there is a difference in output value due to a difference in output characteristics of the inverter devices, as a matter of course, a potential difference as indicated by ΔV in FIG. 12 is generated between the discharge electrodes.
In this way, when there is a potential difference between the discharge electrodes, it is necessary to increase the distance between the discharge electrodes to suppress discharge therebetween; however, this causes a problem of an increase in size of the entire device.
Moreover, if multiple discharge electrodes are kept at a distance, a discharge unit extends over a wide range, and therefore, the area where a noise source and a risk due to an unexpected discharge are to be controlled is expanded.
In view of the above, there is a need to enable an inverter device used in a plasma generating device or the like to apply necessary high voltage to multiple discharge electrodes which are loads without generating an unnecessary discharge between the discharge electrodes.